Rear projection screen displays based on glass microspheres embedded in an opaque matrix as described in U.S. Pat. No. 2,378,252 (Staehle) have been growing in popularity for various uses, such as in large format televisions. A rear projection screen is a sheet-like optical device having a relatively thin viewing layer that is placed at an image surface of an optical projection apparatus. Such a screen makes visible a real image focused by a projection apparatus onto the image surface. The viewing layer is typically planar corresponding to the image surfaces produced by a projection apparatus. Other shapes are possible if the image surface of the projection apparatus is not planar. The screen is intended to act as a filter to attenuate, block, or diffuse light which is not part of the projected image, and to transmit from its rear side to its front side that light which is part of the projected image. In this way it enables the viewer to see the projected image when looking at the front side of the screen.
A well-known type of rear projection screen is a thin, light diffusing layer such as a frosted or translucent glass surface, which may be produced by etching, sandblasting, or otherwise roughening a smooth glass surface. The translucent surface limits the visibility of objects behind the screen. The screen must, however, be sufficiently light transmissive to allow the projected image, which is focused precisely on the translucent surface, to be viewed from the front side of the screen. Since the translucent surface scatters light, the image is viewable from a range of viewing angles. Screens that are merely translucent suffer, however, from a tendency to strongly reflect ambient light incident on the front side, thereby causing fading, or washout, of the projected image. This problem is particularly severe if the background or ambient light is bright.
An approach to reducing the effects of ambient light while still maintaining an acceptable level of projected image light is to attach an array of closely packed glass microspheres (i.e., beads) to a substrate by an opaque polymeric binder. The glass microspheres and substrate are both light transmissible (i.e., transparent). The glass microspheres act as lenses to collect projected light from the rear of the screen and focus it to relatively small spots, near the surfaces of the microspheres. The foci are approximately in the areas where the microspheres contact the front support layer.
Because the transparent microspheres contact the front of the substrate, they exclude most of the opaque binder material from the space between the microspheres and their contact areas on the substrate. This forms an optical aperture between each microsphere and the substrate. The area surrounding each optical aperture is opaque, and preferably black, due to the opaque binder material in the microsphere interstices. As a result, ambient light incident in these areas is absorbed. Thus the front side of the screen appears black, except for the light transmitted through the microspheres.
The appearance of such screens is highly sensitive to the quality and placement of the glass microspheres used. Microspheres that are of incorrect size, are not spherical, or are broken, nicked, scratched, or otherwise defective can create a variety of visible defects, variously called graininess, scintillation, sparkles, speckle, punch through, or simply spots. These defects are particularly troubling when the screen is used, for example, as a computer monitor, where the need for seeing a high level of detail is likely to lead the user to scrutinize the screen closely, from a short distance, for long periods of time.
Generally, the size of the microspheres required for such products are less than about 150 xcexcm and for maximum xe2x80x9cbrightnessxe2x80x9d their index of refraction should be less than about 1.8, and preferably about 1.45 to about 1.75. Higher index microspheres can be employed as taught in U.S. Pat. No. 5,563,738 (Vance); however, to achieve similar brightness values special optical layers are required which adds additional processing steps and cost. It is also taught that it is xe2x80x9cnecessary to eliminate out-of-round, wrong-sized, and discolored microspheresxe2x80x9d in order to obtain a uniform appearance.
A number of processes have been devised for the production of spherical glass bodies in small sizes. These generally involve the free suspension of particles in a hot zone for a time and at a temperature sufficient to permit each particle to be drawn into a spherical shape by surface tension. For economical commercial production of glass microspheres it is important that the viscosity of the glass be relatively low at a reasonable melting temperature (for example, no greater than about 1350xc2x0 C.). Generally, additions of alkali and fluorine are used to reduce the melting temperature; however, the use of fluorine creates an environmental concern as it is readily lost during the melting process and the addition of alkali typically results in microspheres that are hydrophobic and tend to clump and be poorly flowing.
U.S. Pat. No. 2,610,922 (Beck) describes glass compositions suitable for the production of glass microspheres with an index of refraction of 1.64 to 1.74. Compositions that are fluorine-free tend to form fiber when directly atomized from the melt; however, the use of fluorine in the glass results in hazardous emissions which are undesirable.
U.S. Pat. No. 5,716,706 (Morris) describes glass microspheres with a refractive index of 1.6 to 1.9. These glasses are designed to meet the refractive index, chemical durability, and strength requirements of pavement marking applications. These compositions do not readily form small microspheres (e.g., about 150 xcexcm or less) of acceptable quality (e.g., low levels of bubbles) due to the relatively high viscosity at useful microsphere forming temperatures (e.g., about 1350xc2x0 C.).
U.S. Pat. No. 3,306,757 (Duval d""Adrian) describes formulations that can be used to prepare glass microspheres in the desired refractive index range; however, these compositions either require excessive temperatures (e.g., greater than about 1350xc2x0 C.) or are of such a nature that they tend to form fibers when directly atomized from the melt.
U.S. Pat. No. 2,794,301 (Law et al.) describes free-flowing alkali metal oxide containing glass microspheres that are treated with an acidic gas vapor upon manufacture to insolubilize the surface alkali. This process creates undesirable environmental emissions which requires costly control.
Thus, there is a need for free flowing glass microspheres prepared from compositions that have a relatively low melting points and lend themselves to the economical manufacture of glass microspheres. Preferably, there is a need for glass forming compositions that have a low viscosity (e.g., that of vegetable oil) at temperatures no greater than about 1350xc2x0 C. and form microspheres with an index of refraction of no greater than about 1.70, which also have a low level of defects.
The present invention provides glass microspheres and rear projection screens containing glass microspheres, which combine a desirable index of refraction (preferably, no greater than about 1.70, more preferably, about 1.50 to about 1.70, and most preferably, about 1.60 to about 1.70) and low levels of defects (e.g., bubbles, visible haziness, frostiness, or opacity, substantially nonspherical shapes) upon being formed (i.e., xe2x80x9cas producedxe2x80x9d without subsequent sorting to pick out the defects). Preferably, a population of microspheres as produced has less than about 15% defects. The terms xe2x80x9cmicrosphere,xe2x80x9d xe2x80x9cbead,xe2x80x9d and xe2x80x9csphericalxe2x80x9d are used herein for rounded unitary glass elements, which may not be perfect spheres.
Preferably, the glass microspheres are visibly transparent (i.e., they transmit a sufficient amount of light such that they are suitable for use in beaded rear projection screen displays). Microspheres that are suitable for use in displays are preferably less than about 150 xcexcm in diameter. Preferably, the microspheres include, on a theoretical oxide basis and based on the amounts of the starting materials, greater than about 5 wt-% of an alkali metal oxide selected from the group of Na2O, K2O, Li2O, and mixtures thereof, no greater than about 40 wt-% SiO2, and no less than about 10 wt-% TiO2. For specific advantage, the microspheres preferably include Li2O, typically in an amount of at least about 0.25 wt-% Li2O.
For particularly preferred glass microspheres, the components of the glass microspheres are as follows: no greater than about 40 wt-% SiO2; no less than about 10 wt-% TiO2; no less than about 5 wt-% B2O3; no less than about 20 wt-% of an alkaline earth modifier selected from the group of BaO, SrO, and mixtures thereof; and greater than about 5 wt-% of an alkali metal oxide selected from the group of Na2O, K2O, Li2O, and mixtures thereof, preferably with the proviso that Li2O is present. For significant lack of defects, ease of melting, and desirable refractive index, the glass microspheres of the present invention include: no greater than about 31 wt-% SiO2; no less than about 15 wt-% TiO2; no less than about 10 wt-% B2O3; no less than about 25 wt-% of an alkaline earth modifier selected from the group of BaO, SrO, and mixtures thereof; and no less than about 10 wt-% of an alkali metal oxide selected from the group of Na2O, K2O, Li2O, and mixtures thereof.
As is common in the glass art, the components are described as oxides, which is the form in which they are presumed to exist in the completed glass microspheres of the invention, and which correctly account for the chemical elements and their proportions in the glass forming composition. The starting materials used to make the glass may be some chemical compound other than an oxide, such as barium carbonate, but the composition becomes modified to the oxide form during melting of the ingredients. Thus, the compositions of the glass microspheres of the present invention are discussed in terms of a theoretical oxide basis.
The formulations described herein are reported on a theoretical oxide basis based on the amounts of starting materials used. These values do not necessarily account for fugitive materials (e.g., fugitive intermediates) that are volatilized during the melting and spheroidizing process. Typically, boria (B2O3) and alkali metal oxides are somewhat fugitive. Thus, if a finished product were analyzed there could be as much as a 5% loss of the original amount of boria and/or alkali metal oxide added to make the final microspheres. However, herein, as is conventional, all components of the final microspheres are calculated based on the amounts of starting materials and the total weight of the glass forming composition, and are reported in weight percents of oxides based on a theoretical basis.
The present invention also provides a film comprising a plurality of glass microspheres disposed on a substrate and embedded in an opaque matrix; wherein the glass microspheres: have an index of refraction of no greater than about 1.70; comprise, on a theoretical oxide basis based on the amount of starting materials, greater than about 5 wt-% of an alkali metal oxide selected from the group of Na2O, K2O, Li2O, and mixtures thereof (and preferably, Li2O is present), no greater than about 40 wt-% SiO2, and no less than about 10 wt-% TiO2; and as produced have less than about 15% defects in a population. Such films can be used in a rear projection screen.
Also provided is a rear projection screen that includes a plurality of refracting microspheres as described herein bound in optical contact with a substrate and embedded in an opaque matrix. Various embodiments of such screens can incorporate the microspheres of the present invention.
In yet another embodiment, the present invention provides a method of making a film for use in a rear projection screen, which may or may not utilize the glass microspheres described herein. This method includes providing a substrate having an opaque matrix disposed thereon; and applying glass microspheres from a rolling bank of microspheres onto the opaque matrix under conditions effective to produce microspheres in optical contact with the substrate and embedded in the opaque matrix. Preferably, applying glass microspheres from a rolling bank includes: contacting the opaque matrix on the substrate with sufficient glass microspheres to form multiple layers of glass microspheres between the substrate and a pack roll; and pressing the glass microspheres into the opaque matrix on the substrate. Preferably, and advantageously, a monolayer of embedded microspheres is formed, wherein the apex of a majority of the microspheres, and preferably, substantially all the microspheres, are in direct contact with the substrate underlying the opaque matrix.